1. Field of the Invention
The present invention relates to a multicolor image forming apparatus for obtaining color copies by electrophotography in which a latent image formed on a photoreceptor is developed by a plurality of developing means including a multicolor developer.
2. Description of the Related Art
Many methods and apparatuses have been proposed to obtain color images by electrophotography. For instance, Japanese Patent Publication Open to Public Inspection No. 100770/1986 discloses a method for obtaining a color copy which will be explained as follows. Latent images corresponding to the number of decomposed colors of documents, are formed on a photoreceptor. The latent images are transferred onto a transfer drum every time development is conducted and multicolor images are formed on the surface of the transfer drum. After that, they are transferred onto a transfer paper. Color copies are obtained in the way mentioned above. According to this method, the apparatus needs to have a transfer drum having a circumferential surface onto which an image of document size can be transferred. It is unavoidable that the size of the apparatus becomes large and its structure becomes complicated.
For example, Japanese Patent Publication Open to Public Inspection No. 149972/1986 discloses a multicolor image forming apparatus which will be explained as follows. A latent image corresponding to the number of document image basic colors is formed on a photoreceptor and development is conducted. Multicolor copies are obtained by transferring the images onto a transfer paper every time development is conducted. It is difficult to accurately superimpose multicolor images by this method and a color copy with good quality can not be obtained.
There is a multicolor image forming method in which forming a latent image on a photoreceptor corresponding to the number of document image basic colors and development is repeated to superimpose color toner on the photoreceptor, then the color image on the photoreceptor is transferred onto a transfer paper. The basic process of this multicolor image forming method is disclosed in Japanese Patent Publication Open to Public Inspection Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985, and 158475/1985 by the applicants.
In the multicolor image forming apparatus to obtain color images by the method of superimposition mentioned above, a plurality of developing units containing different colored toner are located around the photoreceptor and the photoreceptor generally makes a plurality of revolutions to develop latent images on the photoreceptor, and color images are obtained.
Furthermore, the applicants offered a proposal to improve the work efficiency of a plurality of developing units with the object of simplifying the structure surrounding the photoreceptor of the above-mentioned color image forming apparatus by the applicants to make the apparatus compact and enhance its function. The content of the proposal is disclosed in Japanese patent application No. 89917/1986 which will be explained as follows. A frame which can contact to and detach from the photoreceptor is mounted on the apparatus. When the frame detaches from the photoreceptor, a plate mounted on the frame is pulled out and a developing apparatus mounted on the plate is removed for cleaning and to supply toner.
On the other hand, concerning a copier in general use, a proposal is offered as disclosed in Japanese patent Publication No. 54392/1983. The content of the proposal is as follows. A photoreceptor, a developing unit, and a cleaner are mounted on a support in a body with the object of making it easy to replace and maintain each unit. In the case of monochrome, all of these units are made in a body and they are disposable for easy replacement. That type of apparatus is disclosed in Japanese Patent Publication Open to Public Inspection No. 154255/1982. In the case of a disposable color cartridge, an apparatus in which a plurality of disposable developing units are built, is disclosed in Japanese Patent Publication Open to Public Inspection No. 72159/1983.
However, these apparatuses do not attain the level of the above-mentioned multicolor image forming apparatus with a plurality of developing units of multicolors and full colors in view of the point that the apparatus is compact and excellent in maintenance efficiency.
In the case where a plurality of developing units are located in the peripheral portion of the photoreceptor, toner flies in all directions in the apparatus during operation, more than in the case of a conventional copier. As a result, maintenance work must be conducted frequently.
On the other hand, the above-mentioned multicolor image forming apparatus in which a color image is obtained by superimposing toner images on the photoreceptor, is complicated in structure and large in size. It is especially necessary that the clearance between a plurality of developing units and the surface of the photoreceptor must be accurately maintained.
In order to take measures against these problems, a color process cartridge is considered a solution, wherein the color process cartridge is one body in which a photoreceptor, a plurality of developing units, and furthermore cleaning means are assembled. When a disposable color process cartridge with a plurality of built-in color developing units is to be used among the above-mentioned various kinds of multicolor image forming apparatuses, the color toner consumption of each developing apparatus is different from each other according to the kind of documents to be copied and especially according to the users. Especially the difference of consumption between black toner and other colored toner is remarkable. For that reason, it is disadvantageous that color cartridges with a plurality of built-in color developing units are used as disposable units. There are various kinds of problems in installing a photoreceptor and a plurality of the developing units, explained as follows
(1) In the multicolor image forming apparatus for developing the above-mentioned general color images, superimposed color images, and the like, the arrangement and the shape of each developing unit differ from each other in view of the fact that each apparatus has a different structure. Therefore, when the photoreceptor is replaced, the developing unit is replaced, or when the developing unit is pulled out to replace toner, the photoreceptor is subject to damage. In order to prevent a developing unit of one color from scratching other color toner images developed on the photoreceptor, or to solve the problem of color mixing, a clearance adjusting mechanism must be installed in each developing unit which adjusts the clearance between the photoreceptor and the developing unit. When the bias must be changed, there are problems in the mechanism and in the time. Its structure is complicated and its operation and maintenance work are very troublesome and, furthermore, a great number of parts are needed in the apparatus. As a result, the apparatus tends to lack reliability.
(2) There is a method which has been known as the rotary developer. In this method, the developing unit which is located at a prescribed position of a body of rotation, is rotated by a prescribed angle to develop each color at a fixed position every time each color is developed. In this method, it is difficult to stop the developing unit at a prescribed position with accuracy. Especially in the non-contact developing method, the mechanism to adjust the clearance (D sd) between the photoreceptor and the developing sleeve is complicated and it is hard to set the photoreceptor and the developing sleeve accurately. Furthermore, it takes time to rotate the developing units, therefore it is difficult to conduct development at a high speed because of the restriction of time.
(3) The variation of the development characteristics is one of the factors which degrades the image quality. The variation of the development characteristics is caused by the fluctuation of D sd stemming from the fluctuation of the diameter of the photoreceptor according to the difference of the temperature between the inside and outside of the apparatus, or the fluctuation of D sd according to the vibration of the inside or outside of the apparatus caused by the machines rotation. Actually, the fluctuation of D sd is required to be .+-.0.02 to 0.03 mm. However, the eccentricity of the photoreceptor shaft is usually .+-.0.005 to 0.01 mm. When the photoreceptor is installed in the apparatus, the clearance between the surface of the photoreceptor and the developing unit is .+-.0.1 to 0.3 mm. When the apparatus is in operation, the fluctuation of the temperature inside the apparatus is 5.degree. to 35.degree. C. According to the fluctuation of the temperature, the photoreceptor now in use, made from aluminum expands and contracts by .+-.0.01 to 0.2 mm. The expansion and contraction of the photoreceptor affect the quality of the image, causing mixing and imbalance of color, in combination with the fluctuation of voltage which is caused by the difference of the relationship of positions of the developing unit, the electrifying unit, and the exposure unit.
The main problems to be solved by the present invention are as follows.
(4) Previously, the applicants invented and proposed a method which is disclosed in Japanese patent application Nos. 181792/1986, 89048/1986, and 89917/1986. The method will be explained as follows. As a developing garage, a unit box is installed on a frame on which a plurality of developing units are mounted being integrated into one body. When the individual developing unit is removed, the developing garage on which a plurality of developing units are mounted is once removed from the photoreceptor drum, then the individual developing unit is pulled out from the garage. The problem of this invention is when the individual developing unit is removed, toner is scattered, and scattered toner drops into the apparatus and causes mixing of colors.
(5) The applicants improved the invention which was disclosed in the above-mentioned Japanese patent application Nos. 18172/1986 and 89048/1986, and applied for a patent of Japanese patent application Nos. 234494/1987, 294918, and 320597/1987. In these patents the applicants proposed a color image forming apparatus the details of which will be described as follows. Various process units mainly consisting of a plurality of developing units, a cleaning unit, and the like are mounted on the apparatus equipped with a photoreceptor, together with a slidable frame which is capable of being pulled out and pushed in quickly. Each developing unit which can be dismounted from a toner hopper mounted on the apparatus, can be replaced for maintenance.
However, this invention did not reach a higher level than a disposable color developing unit cartridge in which the individual developing unit is replaced according to the consumption of toner, in order to show consideration for promoting customer's merit and making the apparatus compact.